Stabilized zirconium dioxide is known for a variety of uses, particularly those in which it functions as a solid electrolyte. Such uses include oxygen concentration cells and fuel cells as disclosed in Vielstich's book, Fuel Cells, published by Wiley Interscience (English translation copyrighted 1970), pp. 248-261. Zirconium dioxide ZrO.sub.2 exists in three crystal forms, i.e., the cubic form stable at high temperatures, the tetragonal also stable at high temperature, and the monoclinic which is stable at low temperatures. Since transition between these different crystal habits caused by temperature change result in volume change, parts formed from zirconium dioxide are subject to cracks and breaks when subject to a large temperature change, particularly a rapid change. It is, therefore, necessary and common practice to stabilize the cubic high temperature form completely or partially by adding at least one stabilizing oxide, a number of which are known, for example, CaO, Y.sub.2 O.sub.3, and Yb.sub.2 O.sub.3. Such stabilization minimizes or eliminates the phase changes when subjected to a change in temperature and also the resulting volume changes, so that parts formed from such stabilized zirconium dioxide have substantially greater stability.
It is necessary to sinter zirconium dioxide which is being stabilized by incorporation of one of the said stabilizer oxides at high temperatures because the diffusion of the reacting component is impeded in cubic zirconium dioxide. As a result of this high-temperature sintering, the sintered composition has a coarse-grain structure with unsatisfactory mechanical properties. It is possible to lower the sintering temperature by incorporating small quantities of sintering aids, for example, less than 5 mol %. However, the structure of even such zirconium dioxide is still relatively coarse and, consequently, the mechanical properties are not entirely satisfactory. Alumina, silica, and silicates are examples of such known sintering aids. When silica containing sintering aids are used, the high temperature strength of the zirconium dioxide ceramic product is particularly impaired.
It is known to produce fine-grained zirconium dioxide by incorporating 5 mol % Y.sub.2 O.sub.3 to form a partially stabilized zirconium dioxide which sinters to form dense product at low temperatures. Molded products made from such partially stabilized zirconium dioxide exhibits superior mechanical properties. However, their crystal structure may be damaged by irreversible phase transition from the meta-stable tetragonal zirconium dioxide to the monoclinic zirconium dioxide, particularly when exposed to service conditions having repeated temperature changes. This can ultimately result in fracture of such molded products.
It is an object of the present invention to provide a fine-grained stabilized zirconium dioxide ceramic having superior mechanical properties and oxygen sensors utilizing said zirconium dioxide ceramic.